[MapServer-dev] Call for review: RFC-142: Scalebar Measurement Modes

[Tamas Szekeres]

Hi Devs,

I've prepared an RFC to add explicit measurement modes to the MapServer
SCALEBAR object. The goal is to preserve the current Cartesian scalebar
behavior by default while allowing users to request local geodesic ground
distance measurement for projections where projected map units are not
equal to ground distance.
The main motivating case is Web Mercator / EPSG:3857. In that projection,
coordinates are expressed in meters, but projected meters increasingly
differ from local ground meters away from the equator. The current scalebar
implementation treats projected coordinate units as a flat Cartesian plane,
so metric scalebars can overstate local ground distance in EPSG:3857 and
similar projections.

Links:

- RFC text: https://mapserver.org/development/rfc/ms-rfc-142.html

- Candidate implementation: https://github.com/MapServer/MapServer/pull/7507

- Issue targeted:  https://github.com/MapServer/MapServer/issues/7397

Tamas
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